the string and the cosmic bounce valerio bozza university of salerno, italy

The String and The Cosmic Bounce

Valerio BozzaUniversity of Salerno, Italy

Summary

The Inflation and The String The String and The Bounce The Bounce and The Perturbations The Bounce and The Inflation

Standard Inflation

Why is the universe so flat? Why is the universe so homogeneous on large scales?

1. The Inflation and The String

1aH

K

1aH

k

Inflation can be easily realized using a scalar field dominated by its potential energy.

These puzzles can be solved by invoking a period of accelerated expansion, called inflation, in the very early universe.

The radius of curvature becomes much larger than the horizon

Any inhomogeneities are washed away far beyond the horizon.

CMB and Primordial perturbations

CMB anisotropies today are the outcome of primordial perturbations

k = wave numberF = Newtonian potentialA = amplitudens = spectral index

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012.0012.0

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3.14.10

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4.70

272.0

0456.0

s

R

m

b

n

H

123 snk kAkP

0.2 0.4 0.6 0.8 1Log k

0.2

0.4

0.6

0.8

1

1.2

1.4

Log Pk

ns1

ns1.3

ns0.7


“Unobserved” observables in CMB are


The tensor-to-scalar ratioThe tensor spectral indexNon-gaussianities Running spectral index, oscillations, isocurvature, …

CMB and Primordial perturbations

7410

?

36.0

NL

T

f

n

r

Inflation and primordial spectrum

Whatever the initial conditions, the universe is cleaned out by inflation and all that remains are quantum fluctuations.

241 123 s

nk nkAkP s


The spectrum of quantum fluctuations of the vacuum determines the spectrum of primordial perturbations.

Why look for alternatives to inflation?

A similar spectrum, but with different amplitude, is predicted for primordial gravitational waves.

2 23 T

nTk nkAhkP T

Issues in inflation

Standard inflation provides a remarkable solution to the problems of Big Bang cosmology. Moreover, it yields a prediction for the spectrum of cosmological perturbations in agreement with experimental data.

Yet it leaves some questions unsolved:

It has no roots within any fundamental theory.

(Brandenberger hep-th/0701111)


Initial conditions for perturbations in a Transplanckian regime.Potential very flat (fine tuning?).

It does not answer the initial singularity problem.

String Theory

• String theory unifies all interactions including gravity in a very appealing quantum picture.


• Particles are replaced by strings. There is only one parameter: the string length Ls.

• String theory can be consistently formulated only in 10 (or 11) dimensions.

• There is a huge number of possible low energy limits, depending on the details of the compactification.

• The extra-degrees of freedom survive in the effective 4-D theory in the form of scalar fields (moduli).

• Extra-dimensions must be kept at bay by some compactification mechanism.

String Cosmology

Cosmology can help String Theory to find experimental signatures


String Theory can help cosmology to find a solid foundation for inflation and early universe cosmology.

Two directions

Incorporate inflationin string theory

Alternative “stringy” mechanisms

String Inflation

Inflation can be found in many realizations of string theory


Modular inflation (KKLT , Racetrack, LVS, Kahler, Roulette, …)

(Mulryne & Ward 2011)

Brane inflation (DBI , D3/D7, D-term, …)

Axion (Silverstein & Westpahl ‘08, …)

Tachyon (Sen‘ 05, …)

Higher-derivative (P-adic, CSFT)

Assisted inflation (Liddle et al. ‘98, Dimopoulos et al. ’08, …)M-theory (Becker et al. ‘05, Buchbinder ’05, …)

These models can be already tested or will soon be with PLANCK!

Valerio

The inflaton is a modulus. Compactification and branes dictate the potential

Valerio

Inflation arises as the brane moves in a warped throat

Valerio

Fluxes involved in the compactification details may carry an axion who can play the role of the inflaton

Valerio

A condensate of string on a stack of branes may generate a tachyonic field

Valerio

High-energy corrections to the low energy effective action may be involved in inflation

Valerio

All additional fields of string theory may act collectively and generate an inflation with a large number of fields. Statistics is necessary to make predictions.

Valerio

M-theory can realize inflation before moduli stabilization

String Gas Cosmology

Low energy effective theories are not stringy enough.

2. The String and The Bounce

The very early universe was very different from our low energy world.

Are we missing real string theory?The Universe might have emerged from a gas of strings.

Thermal equilibrium leads here to a scale-invariant spectrum.(Nayeri, et al. ‘06)

The existence of winding modes can explain why we only see 3 spatial dimensions. (Brandenberger & Vafa ‘89)

(Brandenberger et al.)

But… no explicit field theory describing the model!

Supercritical string cosmology

With the “wrong” number of dimensions, a central charge remains


Conformal invariance restored by a “Liouville mode”

Modifications of Boltzmann equations: dark matter spectrum

… more in tomorrow’s talk by Mavromatos

(Ellis, Mavromatos, Nanopoulos 1993;Gravanis &Mavromatos 2002)

The BounceThe string length provides a fundamental cut-off for quantum gravity.

t

HtPreBounce

PostBounce

t

at

PreBounce PostBounce


The Big Bang singularity should be cured by string theory.

If there is no singularity, what happened before the big bang?

The present expansion was perhaps preceded by a contraction phase ending with a cosmic bounce.

Horizon and Flatness

The spatial curvature becomes negligible as we approach the bounce:

Perturbations never become Trans-Planckian!

02 a

aK

a

K

dt

d

aH

K

dt

d


Since there is no singularity in the past, there is no particle horizon:

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lim if -t

)(ta

ta

dtt

Challenges for bouncing cosmologies

• A cosmic bounce requires violation of the Null-Energy-Condition.Ghosts? Instabilities?


• Calculating the evolution of perturbations across the bounce is absolutely non-trivial!

• The correct perturbation spectra must be generated.

• Anisotropies grow during contraction. Super-stiff source required (w>1)

The Pre-Big Bang scenario is based on considerations about the T-duality of string theory.

Pre – Big Bang scenario(Veneziano, Gasperini 1991)


The spectrum of the Bardeen potential after the bounce is steeply blue (ns=4).

The spectrum of the axion is scale-invariant curvaton mechanism

aa

1

(VB, Gasperini, Giovannini & Veneziano 2002).

Bounce with time-like extra-dimension (Shtanov & Sahni 2003)

Bounces with extra-dimensions


Bounce of a probe brane (Brax & Steer 2002; Burgess et al. 2003; Kachru & Mcallister 2002; Germani et al. 2006; Easson et al. 2007)

Bounce of a boundary brane (Mukherji & Peloso 2002; Burgess et al. 2003)

Quantum cosmology bounces

•Loop Quantum Cosmology (Bojowald, Maartens & Singh 2004; …)

•Wheeler – de Witt approach (Peter, Pinho & Pinto-Neto 2006)

•Quantum backreaction (Srivastava 2007)

High-energy Bounces


Higher derivative bounces

•’ corrections of string theory (Tsujikawa, Brandenberger & Finelli 2002)

•Non-perturbative corrections (e.g. exp()): no ghosts arise (Biswas, Mazumdar & Siegel 2005)

Non-local dilaton potential (Gasperini, Giovannini & Veneziano 2003)

Low-energy Bounces


Ghost condensate (Creminelli, Luty, Nicolis & Senatore 2006)

Weyl Integrable Spacetime duality (Novello et al. 1993)

Non-minimal coupled Vector Fields(Novello & Salim 1979)

In the Ekpyrotic scenario, the visible universe is a brane embedded in an11-dimensional space-time.

The bounce occurs when our brane collides with a hidden brane (singularity!).

The observer on the visible brane sees a scalar field with a negative exponential potential

We have w>>1 and the scale factor is then where p can be chosen arbitrarily small and positive.

(Steinhardt, Turok et al. 2001)

Visible

universe

brane

Hidden

brane

Bulk

p

P

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1

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Ekpyrotic/Cyclic Universe2. The String and The Bounce

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2.5

5

7.5

10

12.5

15

A

ns1

ns5

2 1 1 2Log a

6

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ns1

During a slow contraction, scalar perturbations grow with a scale-invariant spectrum.

After the bounce we have two possibilities:

The growing mode delivers its spectrum to a constant mode.

The growing mode turns into a decaying mode, leaving place to a blue constant mode.

Log

Log

Ekpyrotic/Cyclic Universe3. The Bounce and The Perturbations

4 2 2 4 6Log a

2.5

5

7.5

10

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15

A

ns1

ns5

2 1 1 2Log a

6

8

10

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A

ns1

What decides whether the growing mode survives or

not?

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Log

Perturbations through the bounce

3. The Bounce and The Perturbations

General solution for scalar perturbations

We base our general analysis on four minimal assumptions:

(VB and Veneziano 2005; VB 2006)

TG

31w


4) Before and after the bounce, the universe is characterized by constant w and cs

2, with (no inflation, no deflation).

3) The bounce is entirely determined by a unique physical scale.

2) The universe is homogeneous and isotropic; thus the background metric is FRW.

1) It makes sense to define a 4-dimensional metric tensor at all times. Then we can always write effective Einstein equations .

up

2up

4 2 2 4 6Log a

2.5

5

7.5

10

12.5

15

A

ns1

ns5

2 1 1 2Log a

6

8

10

12

A

ns1

Log

Log

General solution for scalar perturbations

(VB and Veneziano 2005; VB 2006)


Relation to other works

Our conclusions are in agreement with the hypersurface analysis by Durrer & Vernizzi (2002): the tension of the bounce hypersurface is related to the pressure perturbation.Is fine-tuning on the bounce?

Creminelli, Nicolis & Zaldarriaga (2004) study the attractor nature of the Ekpyrotic solution in the synchronous gauge.Is fine-tuning on the bounce?

Chu, Furuta & Lin (2006) have connected the transmission of the Pre-Bounce spectrum with the presence of non-local microphysics.

00 G

00 G


valerio

This does not apply to a regular bounce, in which the synchronous gauge becomes singular when the NEC violation occurs.

Isocurvature perturbations(Lehners, McFadden, Steinhardt & Turok 2007)

A subdominant scalar field with a negative exponential potential develops a scale-invariant spectrum.


By transferring this isocurvature spectrum to the Bardeen potential at the onset of the bounce, the spectrum after the bounce remains scale-invariant.

The isocurvature generation can be associated with a regular bounce model (for example the ghost condensate):

“New Ekpyrotic cosmology”

(Buchbinder, Khoury & Ovrut 2007 – Creminelli & Senatore 2007)

Large non-gaussianities, fine-tuning, pre-ekpyrosis!

“Dust”-dominated Pre-Bounce(Wands 1999; Finelli & Brandenberger 2002;Cai et al. 2008, 2009)

If the contraction is dominated by a source with , the growing mode has spectral index

If the growing mode dies out in the post-bounce, the final constant mode has spectral index

To get one may choose w_=0 (anisotropy problem, unstable!)

wp

wns 31

41

wns 31

45

1sn


Non-linear EoS tames anisotropy

In radiation-dominated contraction, anisotropies can be heavily suppressed.

Even for a dust-like contraction (w=0), we get a small suppression

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c

ii

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T

0,

82

2

2

110

Non-linearities in EoS can help keeping anisotropy under control.


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Suppose the EoS can be Taylor-expanded as

(VB & Bruni 2009)

Comparing predictions

Inflation

4. The Bounce and The Inflation

Bounce

Solves horizon and flatness Solves horizon and flatness

Solves anisotropies Solves anisotropies*

241 sn 1 sn

2 Tn 0 Tn

9 r 1 r

Black hole overproduction?

Circles in the sky?(Gurzadyan & Penrose

2011)

Conclusions

• String theory suggests a resolution of the Big Bang singularity

• Even if inflation is the correct paradigm, it needs a UV completion to be fully satisfactory (singularity, Transplanckian problems)• Predictions are however possible and testable with CMB

• Bouncing cosmologies still face many problems (bounce details, perturbations, anisotropies)

• String Theory may accommodate inflation and provide an answer to all these problems

the string and the cosmic bounce valerio bozza university of salerno, italy

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